The invention relates to a gas flow switching device for switching gas flows between gas sources and gas sinks. The gas flow switching device includes gas passages that communicate with one another and connecting points for the gas sources and gas sinks. Furthermore, the gas flow switching device includes a device for setting different pressures at predefined connecting points.
German Patent DE 28 06 123 C2 teaches a gas flow switching device that is used to change gas flow directions in a chromatographic separation column switching system. Therein, pressure drops of changing direction are generated between suitable points in the separation column switching system. To this end, the known gas flow switching device includes a main gas passage having two connecting points, which is disposed between two separation columns. In the vicinity of each of the two connecting points, the main gas passage is connected with a respective auxiliary gas passage via a respective connecting gas passage. The two auxiliary gas passages are connected with a carrier gas source via a device that comprises several valves for setting different pressures. By setting different pressure drops between the auxiliary gas passages themselves and between the auxiliary gas passages and the connecting points of the main gas passage, a gas sample exiting from the first separation column may enter the second column or may be prevented from entering the second column. Therein, the latter event occurs in the operating mode xe2x80x9ccut.xe2x80x9d In this case, the gas sample is directed to a downstream detector or to a third separation column via the corresponding auxiliary gas passage. Furthermore, the first separation column with the carrier gas may be backflushed from the carrier gas source. The valves required for switching the gas flows come into contact with the carrier gas only but not with the gas sample. Moreover, the valves can be disposed outside the oven that is typically used to heat the separation columns.
For implementing the gas passages, the known gas flow switching device has a block with a center bore into which the end pieces of the two separation columns are inserted from both sides. The main gas passage includes a capillary, which extends coaxially in the center bore and whose ends project into the end pieces of the separation columns. The auxiliary gas passages are embodied as capillaries, which are inserted into the block and which lead into two spatial halves of the center bore. Therein, the spatial halves are sealed against one another. The connecting gas passages are formed by the annular gaps between the end pieces of the separation columns and by the capillary of the main gas passage that projects into the separation columns. The multipart construction of the known gas flow switching device is thus relatively complex. In addition, the parts of the known gas flow switching device must be calibrated in relation to one another.
European Patent EP 0 386 033 B1 teaches a further gas flow switching device, which is used for a valve-less metering of a gas sample for gas chromatographic analysis purposes. For this purpose, a carrier gas passage and a sample gas passage, which communicate with one another via a connecting gas passage, are connected to a carrier gas source via a device for setting different pressures. Therein, a metering device is disposed between the carrier gas source and the sample gas passage for injecting a sample gas slug into the carrier gas flow. The sample gas passage has the form of a tubular chamber. The carrier gas passage includes two interior tubes of different diameters, which penetrate the chamber and whose ends are pushed into one another so as to form an annular gap. This annular gap represents the connecting gas passage between the sample gas passage and the carrier gas passage. By adjusting different pressures in the carrier gas passage and in the sample gas passage, the sample gas from the sample gas passage is prevented from entering the carrier gas passage at the location of the annular gap. Alternatively, the sample gas from the sample gas passage may be specifically channeled into the carrier gas flow flowing through the carrier gas passage. In this known gas flow switching device too, the multipart construction is comparatively complex.
It is an object of the invention to simplify the construction of a gas flow switching device, wherein precisely defined pressure and flow conditions are to be achieved without the need for calibration.
This and other objects of the invention are achieved by a gas flow switching device for switching gas flows between gas sources and gas sinks. The gas flow switching device according to the invention includes two plates, which are positioned on top of one another and which are joined together. These plates have congruent channels on those sides of the two plates that face each other. The congruent channels have semicircular cross sections and form gas passages that communicate with each other. Furthermore, the congruent channels form connecting points for the gas sources and the gas sinks at points at which the congruent channels exit the two plates. In addition, the gas flow switching device according to the invention includes a device for setting different pressures at predefined ones of the connecting points.
The channels are produced in the plates with great technological precision. Therefore, the desired pressure and flow conditions, for which the geometries of the gas passages are calculated, can actually be obtained in practice. In contrast to the parts of the known gas flow switching devices, the plates of the gas flow switching device according to the present invention are comparatively easy to calibrate, and the joining of the plates is done automatically or semi-automatically. Finally, the planar structure of the gas flow switching device according to the invention is highly compact. Very small dimensions are obtained, particularly if the gas passages are produced micromechanically.
The gas sources and gas sinks are preferably connected with the gas passages of the gas flow switching device via capillaries. To this end, the cross sections of the channels at the connecting points are larger than the cross sections of the channels in the area of the gas passages in between the connecting points, and the capillaries, together with their ends, are inserted into the connecting points. Therein, the cross sections of the areas of the gas passages located directly behind the connecting points correspond to the inner cross sections of the capillaries to prevent the creation of flow impediments.
The channels in the plates may principally be made in various ways, e.g., by means of a laser. The plates are preferably made of monocrystalline silicon in which the channels are formed by isotropic etching. This is done, for instance, by means of a mixture of hydrofluoric acid and nitric acid. Alternatively, in the area of the channels, the monocrystalline silicon may be converted into porous silicon and subsequently removed by etching. The etching process in the porous silicon is isotropic, so that the channels formed therein have the desired semicircular cross sections. The channels may be lined with a silicon dioxide layer in order to protect them against the flowing gas.
To switch sample gas and carrier gas flows between two chromatographic separation columns, as it is known from the aforementioned German Patent DE 28 06 123 C2, the channels in the plates of the gas flow switching device according to the invention form a main gas passage, two auxiliary gas passages and two connecting gas passages. Moreover, a respective auxiliary gas passage extends on either side of the main gas passage. Each of the two auxiliary gas passages is connected to the main gas passage via one of the connecting gas passages. The junction points of the connecting gas passages to the main gas passage are mutually offset along the main gas passage. The cross sections of the connecting gas passages are smaller than the cross sections of the main gas passage and the cross sections of the auxiliary gas passages. The cross section of the main gas passage in the area between the junction points of the connecting gas passages is smaller than the cross section outside that area. In a generally known manner, the main gas passage is series-connected to the two separation columns between these columns, and the auxiliary gas passages on one side are connected to a carrier gas source via the device for setting different pressures. In order to measure the different pressures or pressure drops between the auxiliary gas passages, which are required for switching the gas flows between the separation columns, each of the auxiliary gas passages is advantageously connected to connecting points for pressure measuring devices via branching gas passages.
To meter a sample gas, particularly for gas chromatographic analysis purposes as disclosed in the German laid-open publication DE 37 35 814 A1, the channels in the plates of the gas flow switching device according to the invention form a carrier gas passage, a sample gas passage, as well as a connecting gas passage between the carrier gas passage and the sample gas passage. At the branch of the connecting gas passage from the sample gas passage, the cross section ratio of the connecting gas passage and the continuation of the sample gas passage corresponds to a predefined dividing ratio of the sample gas flow. In a generally known manner, the carrier gas passage and the sample gas passage are, on one side, connected to a carrier gas source via the device for setting different pressures. A metering unit for injecting a sample gas slug into the carrier gas flow is disposed between the carrier gas source and the sample gas passage. By determining the cross section of the connecting gas passage and the cross section of the continuation of the sample gas passage as a function of the adjusted division of the sample gas flow, discrimination between differently sized gas molecules is prevented when a portion of the sample gas is diverted from the sample gas passage into the connecting gas passage. Large molecules, e.g., molecules of the sample gas, are not as easily deflected as smaller molecules, e.g., molecules of the carrier gas. Consequently, if the branching fork is asymmetrical, either the larger or the smaller molecules of the sample gas would be more likely to reach the connecting gas passage and, subsequently, the carrier gas passage. This would distort the measurements in the subsequent gas chromatographic analysis. In a preferred 50:50 split of the sample flow, the branching fork of the connecting gas passage from the sample gas passage is symmetrical.
As previously mentioned, the gas passages in the gas flow switching device according to the invention can be formed with great accuracy by means of micromechanical production methods, so that the geometry of the gas passages after production is very precisely known. This is advantageously exploited in that the gas flow switching device according to the invention, together with at least one chromatographic column connected thereto, includes the device for setting different pressures, wherein the device has electronic pressure regulators. The set point values of these pressure regulators are calculated and set based on geometric data of the gas passages and the separation column. In addition, these set point values are calculated and set as a function of the gas flow parameters and as a function of the temperature and the desired flow rate in the separation column. This eliminates the previously required basic pressure calibration by means of adjustable needle valves.
Since, as a rule, the inner diameter of the separation column is not exactly known due to manufacturing tolerances and due to the coating of the column with a liquid separation phase, the gas flow switching device is preferably operated with a test gas or sample gas, when the calculated set point values are set at the pressure regulators. Therein, the transit time (retention time) of the sample gas through the separation column is measured and, based thereon, the average inside diameter of the column is calculated. The sample gas is e.g. air, which does not interact with the separation phase of the column. Based on the thus determined average inner diameter of the separation column, the set point values for the pressure regulators are then recalculated and reset.